The invention relates to a process for operating an internal combustion engine as specified in the preamble of claim 1 and to a device for application of the process.
In the case of internal combustion engines, lean-mix operated Otto engines in particular, compliance with exhaust gas regulations requires reduction of the nitrogen oxide (NOx) component. Either NOx storage catalytic converters or DeNOx storage catalytic converters are used in the exhaust system. NOx storage catalytic converters store the NOx present in exhaust gas and release it under certain operating conditions (xcex less than 1). Desorption of the NOx on the surface of the NOx storage catalytic converter at certain intervals is necessary, since the storage capacity of NOx storage catalytic converters is limited. In order to remain within the exhaust gas limits during NOx regeneration, in the downstream catalytic converter component the NOx released is reduced and the reduction agents HC and CO which are not completely converted are oxidized. A device such as this is disclosed in EP 560991.
One disadvantage of the state-of-the-art storage catalytic converters is that their NOx storage capacity decreases over time as a result of sulfur deposits, chiefly in sulfate form and the operating efficiency and as a result the operating efficiency of the entire exhaust gas system is significantly impaired. It is not possible to remain within exhaust gas limits with xe2x80x9csulfur-contaminatedxe2x80x9d NOx storage catalytic converters. Sulfur deposits are determined by the sulfur fraction of fuel.
Desorption of sulfur from the surface of the NOx storage catalytic converter is known to be theoretically possible. This process is designated as desulfurization in what follows. A prerequisite for the process is that the NOx storage catalytic converter be at a specific temperature and that a reducing environment (sufficient HC and CO) be present at the same time.
However, these conditions can be fulfilled only after prolonged operation of the internal combustion engine under full load (or high partial load). Such an operating condition is vehicle-dependent or driver-dependent and thus highly unpredictable. It depends on a large number of conditions such as driver intent, traffic and road conditions, etc.
Along with the point in time, the duration of this mode of operation and thus the period of operation available for desulfuration are of course also constantly unpredictable.
Desulfuration of a NOx storage catalytic converter on the basis of full-load or high partial-load operation is not possible for every vehicle with the regularity required.
Hence it is possible that the operating efficiency of a NOx storage catalytic converter may be greatly impaired by sulfur deposits and that the NOx component of exhaust gas will rise sharply as a result.
The type of desulfuration of a NOx storage catalytic converter described above presents a number of disadvantages. Such desulfuration is extremely irregular and can be carried out only under very special operating conditions (full load or high partial load) accompanied by increased fuel consumption. While it is true that the condition of the NOx storage catalytic converter is monitored during operation, it is not possible to interpret the results to determine the origin of increased NOx emissions. In addition, a driver may be directed to drive under full load, but only rarely can this be realized.
The object of the invention is to develop a process of operating an internal combustion engine which does not present the disadvantages indicated in the foregoing, a process by means of which desulfuration of a NOx storage catalytic converter is possible at all times, which requires no costly additional equipment for its application, and which is simple and cost effective in application. Another object of the invention is to develop a device for application of the process.
This object is attained by means of the features indicated in claims 1 and 13.
The essential idea of the invention is that, with a requirement set for desulfuration of the NOx storage catalytic converter, in an initial time interval T1 operation of the internal combustion engine is set to a value xcex less than 1 and at the same time secondary air is blown in upstream from the NOx storage catalytic converter in order to heat this catalytic converter. In a subsequent time interval T2 the secondary air injection upstream from the NOx storage catalytic converter is interrupted and the internal combustion engine is operated at a value xcex2 less than 1. Desorption proper of the sulfur on the surface of the NOx storage catalytic converter takes place during this second time interval T2, since an adequate reducing atmosphere is now present.
The process corresponding to time intervals T1 and T2 may be repeated several times in succession. Normal operation of the internal combustion engine is resumed when the desulfuration has been completed.
The essential advantage of the process claimed for the invention is represented by the circumstance that desulfuration is carried out automatically, the necessary measures can be carried out over a broad range of operating conditions of the internal combustion engine, and at the same time the process has no noticeable effect on the performance of the internal combustion engine. The driver will not notice the desulfuration process.
Advantageous developments of the invention are indicated in the subsidiary claims.